iPhone : How to detect the end of slider drag?

I use the "Touch Up Inside" and "Touch up outside" notifications.

Interface Builder:

Connect both notifications in the Interface Builder to your receiving method. The method could look like this:

- (IBAction)lengthSliderDidEndSliding:(id)sender {
    NSLog(@"Slider did end sliding... Do your stuff here");
}

In code:

If you want to wire it programatically you would have something like this in your viewWillAppear (or wherever it fits you) call:

[_mySlider addTarget:self
              action:@selector(sliderDidEndSliding:) 
    forControlEvents:(UIControlEventTouchUpInside | UIControlEventTouchUpOutside)];

The receiving method would look like this:

- (void)sliderDidEndSliding:(NSNotification *)notification {
     NSLog(@"Slider did end sliding... Do your stuff here");
}

You can add an action that takes two parameters, sender and an event, for UIControlEventValueChanged:

[slider addTarget:self action:@selector(onSliderValChanged:forEvent:) forControlEvents:UIControlEventValueChanged]

Then check the phase of the touch object in your handler:

- (void)onSliderValChanged:(UISlider*)slider forEvent:(UIEvent*)event {     
    UITouch *touchEvent = [[event allTouches] anyObject];
    switch (touchEvent.phase) {     
        case UITouchPhaseBegan:
            // handle drag began
            break;
        case UITouchPhaseMoved:
            // handle drag moved
            break;
        case UITouchPhaseEnded:
            // handle drag ended
            break;
        default:
            break;
    }
}

Swift 4 & 5

slider.addTarget(self, action: #selector(onSliderValChanged(slider:event:)), for: .valueChanged)

@objc func onSliderValChanged(slider: UISlider, event: UIEvent) {
    if let touchEvent = event.allTouches?.first {
        switch touchEvent.phase {
        case .began:
            // handle drag began
        case .moved:
            // handle drag moved
        case .ended:
            // handle drag ended
        default:
            break
        }
    }
}

Note in Interface Builder when adding an action you also have the option to add both sender and event parameters to the action.

If you don't need any data inbetween drag, than you should simply set:

[mySlider setContinuous: NO];

This way you will receive valueChanged event only when the user stops moving the slider.

Swift 5 version:

mySlider.isContinuous = false